The present invention relates to material for temperature sensitive elements or parts, and particularly to a material for temperature sensitive elements consisting of a ferromagnetic material of a rare earth cobalt compound of which the magnetic anisotropy varies depending upon the temperature.
When a ferromagnetic body of a rare earth cobalt compound is rotatable and is positioned between two permanent magnets 2a and 2b, as illustrated in FIG. 1, the ferromagnetic body 1 turns toward a fixed direction against the magnetic field generated by the permanent magnets 2a and 2b, due to the magnetic anisotropy of the ferromagnetic body 1. As the ferromagnetic body 1 is gradually heated, the body 1 of some kinds of rare earth compounds does not rotate, but the body 1 of other kinds of rare earth compounds starts rotating at a temperature of T.sub.1, rotates by an angle of 90 degrees, and stops at a temperature of T.sub.2. The rotation phenomenon of the ferromagnetic body is generated by variation of the easy direction of magnetization of the body by an angle of 90 degrees due to the spin reorientation depending upon temperature.
The variance of the direction of easy magnetization of the rare earth cobalt compound will now be explained in detail.
RCo.sub.5 type compounds (R being a rare earth element) have the crystal structure of the hexagonal system, as illustrated in FIG. 2a. In FIG. 2a, the small circle indicates the cobalt element and the large circle having dots indicates the rare earth element. When the direction of easy magnetization of the RCo.sub.5 type compound is parallel to the c-axis ([0001]direction) of the crystal, the state is indicated by the symbol "A" in FIGS. 2b and 3. When the direction of easy magnetization is in the basal plane ((0001)plane) of the crystal, the state is indicated by the symbol "P" in FIGS. 2b and 3. When the direction of easy magnetization is present between the c-axis and the basal plane, for example on a surface of an imaged cone, the state being intermediate between the A state and P state is indicated by the symbol "C" in FIGS. 2b and 3. Temperature dependence of the direction of easy magnetization of RCo.sub.5 type rare earth cobalt compounds is shown in FIG. 3 (cf. the Bulletin of the Japan Institute of Metals, Vol. 16, No. 2, 1977, page 83).
As is obvious from FIG. 3, when the rare earth element is praseodymium (Pr), neodymium (Nd), terbium (Tb) or holmium (Ho), the direction of easy magnetization varies, depending upon temperature. Particularly, the direction of easy magnetization of NdCo.sub.5 and TbCo.sub.5 can vary from the P state to the A state via the C state. As to the rest of the RCo.sub.5 type compounds, the direction of easy magnetization is constant in the A state. The broken lines in FIG. 3 denote the undetermined or presumed state of the direction of easy magnetization.
As to the R.sub.2 Co.sub.17 type rare earth cobalt compounds, temperature dependence of the direction of easy magnetization is shown in FIG. 4 (cf. the same page of the above mentioned reference). In FIG. 4, the symbols A, C and P and the broken lines have the same meaning as explained above. The direction of easy magnetization of the Lu.sub.2 Co.sub.17 compound only can vary from the P state to the C state. There is no R.sub.2 Co.sub.17 type compound of which the direction of easy magnetization can vary from the P state to the A state via the C state.
The direction of easy magnetization of Y.sub.1-x Nd.sub.x Co.sub.5 compound varies depending upon temperature, as illustrated in FIG. 5, when the molar ratio parameter "x" is 0.25, 0.50, 0.75 and 1. In FIG. 5, the symbol ".beta." indicated at the ordinate means the angle between the c-axis of the crystal and the direction of easy magnetization. As can be seen from FIG. 5, a transition temperature range wherein the angle .beta. varies from 90 degrees to zero degrees (i.e. the direction of easy magnetization varies from the P state to the A state) can change, depending on the composition of the rare earth elements (i.e. the molar ratio "x"). In this case, for example, the transition temperature range of NdCo.sub.5 ("x" being 1) is from 230.degree. to 285.degree. K. (i.e. from -43.degree. to 12.degree. C.).
Furthermore, the direction of easy magnetization of the DyCo.sub.z compound varies depending upon temperature, as is illustrated in FIG. 6, when the molar ratio parameter "z" is 4.4, 4.6, 5.0 and 5.3. As can be seen from FIG. 6, the transition temperature range can be changed, depending the composition of the dysprosium cobalt compound (i.e. the molar ratio "z"). The data of FIG. 6 were obtained as a result of the present inventor's experiments. Test pieces of DyCo.sub.z compounds were produced in accordance with the process for producing a magnetic body proposed by the present inventors as U.S. Pat. Nos. 4,347,201 and 4,459,248 (European Patent Application No. 79302389.6 i.e., EP-A-0010960). The process is disclosed in column 6, lines 10-18, and 50-52, and column 10, lines 20-17, of U.S. Pat. No. 4,347,201, and in column 6, lines 11-19 and 51-53 and column 10, lines 20-27, of U.S. Pat. No. 4,459,248). The DyCo.sub.z compound has a disadvantage, i.e. a relative low saturation magnetization, as shown in Table 1, therefore, when the DyCo.sub.z compound body is used as a switch element of a temperature sensitive device, the switching property of the switch element is low so that the device has a disadvantageously large size.
TABLE 1 ______________________________________ Saturation Magnetization (T) Material at Room Temperature ______________________________________ DyCo.sub.5 0.437 NdCo.sub.5 1.228 TbCo.sub.5 0.236 Thermorite* 0.26 Magnetic Shunt Alloy** 0.24 ______________________________________ *Mn--Zn system ferrite having a Curie point of 90.degree. C.; **Fe--Ni system alloy steel having a Curie point of 50.degree. C.;
As can be seen in Table 1, the saturation magnetization of a NdCo.sub.5 compound is the largest among the RCo.sub.5 compounds of which the direction of easy magnetization can vary from the P state to the A state via the C state.